FM cochannel interference cancellation systems are well known. The general problem of cochannel interference arises when two FM signals are so close in frequency as to interfere with each other, such as may occur in fringe areas between FM stations, or by deliberate jamming in communications systems. In a typical FM tuner, only the stronger of the two FM signals will be captured by the FM receiver. Furthermore, if the weaker signal is only slightly weaker than the stronger, a distorted stronger signal will result.
U.S. Pat. No. 3,911,366 to Baghdady discloses a system for suppressing interfering FM signals. The basic principle is to receive the undesired FM signal (typically the stronger FM signal), track its instantaneous frequency, and then control an adaptive notch filter to track and remove the undesired FM signal. Thus, the stronger FM signal can be removed. One of the keys to effective realization of a cochannel interference cancellation system is the implementation of the adaptive notch filter.
A fixed notch filter removes signal energy contained in a narrow band of frequencies. The fixed frequency of the notch is determined by the value of the circuit elements, as is well known to those skilled in the art. An adaptive notch filter which has an electrical control input to vary the center frequency of the notch, is more complex and difficult to implement than a fixed frequency notch filter. The adaptive notch filter shown by Baghdady uses a fixed frequency notch filter and two mixer circuits to create the effect of a controllable adaptive notch filter.
To realize an adaptive notch filter, the input signal is translated in the frequency spectrum, in a first mixer circuit, in a dynamic manner so that the frequency of the undesired FM signal is always centered on the fixed notch frequency of the notch filter. After the fixed frequency notch filter, the signal is translated in the frequency domain in a second mixer circuit back to its original position in the frequency spectrum.
The principle disadvantage of the adaptive notch filter disclosed by Baghdady, is that the system requires an accurate fixed frequency oscillator precisely equal to the center frequency of the notch. Specifically, the filter control signal is first heterodyned with the fixed frequency oscillator signal, then filtered to remove an unwanted difference term, in order to form the intermediate control signal which is then used as the local oscillator input to the two mixer circuits. Since the fixed center frequency of the notch must correspond exactly to the frequency of the fixed frequency oscillator, a notch filter with a very small stop band cannot be used because a small error in the instantaneous frequency of the local oscillator control signal will cause the notch stop band to miss the FM signal to rejected. Also, the analog approach of Baghdady does not lend itself to integrated circuit techniques.